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P25 Linear Simulcast Modulation ASTRO

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SlipNutz15

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Ok, I may be overthinking this but Centre County 911's new P25 radio system has three emissions,

8K10F1D P25 Phase I C4FM data
8K10F1E P25 Phase I C4FM voice (P25 mode in RadioReference.com Database)

and then

8K70D1W P25 Linear Simulcast Modulation ASTRO (9.6 kbps in 12.5 kHz channelspace)



I understand the first two. What exactly is the third one? What does it do that the first two do not?
 

GTR8000

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8K70D1W is the GTR 8000 repeaters transmitting LSM (CQPSK) modulation.

The other two emission designators are for the subscriber units, which transmit C4FM modulation.

You'll often see 8K10F1D/8K10F1E listed for the repeaters also, since they technically are capable of transmitting C4FM modulation, even if they never will under normal operating circumstances.
 

SlipNutz15

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If I read correctly on another website the CQPSK is 6.25 instead of 12.5 giving it faster throughput?

The field radios can or cannot decode the CQPSK?
 

SlipNutz15

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Ok, I looked further. For whatever reason I missed it the first time around. All the towers and mobiles have all three emissions. Does that mean they operate 6.25 across the board?
 

jim202

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Ok, I may be overthinking this but Centre County 911's new P25 radio system has three emissions,

8K10F1D P25 Phase I C4FM data
8K10F1E P25 Phase I C4FM voice (P25 mode in RadioReference.com Database)

and then

8K70D1W P25 Linear Simulcast Modulation ASTRO (9.6 kbps in 12.5 kHz channelspace) I understand the first two. What exactly is the third one? What does it do that the first two do not?

I gather that the system your talking about has "SIMULCAST" being used. What this means is that the different tower sites are all using the same frequency to transmit on. This causes some very tight tolerances in the transmitter frequency at each tower site. It also requires that the transmitter be very linear in the modulation being transmitted. If not, then the audio that is reproduced at each receiver can become distorted and as such hard to understand.

Another requirement is the time delays between the audio coming from each tower site has to be held to very tight control. If audio delays start to creep in, it can cause cancellation of the audio due to being out of phase. Look at it as multi path. If you get just the right phase shift, the signals can cancel out each other. This normally happens in the no capture zones that are mid way between the different sites in what we call the overlap zones. No one transmitter signal is stronger than the other. So your receiver has trouble trying to decode the transmission unless the audio delays and the transmitter frequency is tightly controlled.

Hope this has made the picture a little easier to understand.


I understand the first two. What exactly is the third one? What does it do that the first two do not?[/QUOTE]
 

SlipNutz15

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Right. It is a P25 Digital Trunked system. The initial day of testing the outer region towers' timing was synced up. I'm guessing the center region towers are syned up too. Half the towers are using the same group of frequencies and then the other half are using a different frequencies. I'm guessing the group as a whole would be told to transmit at the same time based off the controllers?
 

SlipNutz15

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Ok after I actually woke up I realize I misspoke. The ASTRO simulcast is for the trunk system and then the 9.6 is where the 9600 baud comes in correct?
 

GTR8000

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ASTRO 25 systems use 9600 baud control channels like all P25 systems. If the system is simulcasted, then the linear repeaters employ CQPSK modulation, which Motorola calls "LSM" (Linear Simulcast Modulation). Only the fixed network equipment, i.e. the GTR 8000 repeaters, transmit in CQPSK/LSM modulation. The subscriber units, i.e. the radios, receive and decode the CQPSK/LSM modulation from the repeaters. When the subscriber units transmit, they use C4FM modulation, not CQPSK/LSM. P25 systems operate within 12.5 kHz channel spacing, not 6.25 kHz. Phase II systems divide the 12.5 kHz space into two time slots using TDMA, which achieves 6.25 kHz efficiency, not true 6.25 kHz bandwidth.
 
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